Outdoor antenna for an intrusion detection system



Oct. 28, 1958 H. PEARSON 2,853,359

OUTDOOR ANTENNA FOR AN INTRUSION DETECTION SYSTEM Filed Feb. 4, 1957 |2 l3 14 F l 6 l osc. 1:25 aeuw POWER SUPPLY a United States Patent OUTDOOR ANTENNA FOR AN INTRUSION DETECTION SYSTEM Howard Pearson, New York, N. Y., assignor to American District Telegraph Company, Jersey City, N. J., a corporation of New Jersey Application February 4, 1957, Serial No. 637,956

2 Claims. (Cl. 174139) This invention relates to intrusion detection systems and more particularly to systems in which perimeter wire functions as an antenna to sense the presence of an intruder due to a small change in coupling between wires or between the wire and ground.

Such systems are usually balanced so that a predetermined and relatively rapid change in coupling such as in the capacitance between the antenna wire and ground caused by the approach of an intruder and the concomitant change in antenna circuit current triggers an alarm circuit which may be used to operate well known types of signal equipment such as annunciators or the like.

A serious drawback in the operation of such systems where the antenna wire is installed outdoors resides in the fact that hitherto wetting of the wires and their supports by rain has caused a large and rapid change in the coupling between the wires themselves or between them and ground. The glass and porcelain insulators commonly used to attach the wires to their supports have a large surface area and are in intimate contact with the wire with the result that a significant and abrupt change in capacitance occurs when the parts of the system are wet by rain. In fact, when only a single one of such insulators becomes wet, the capacitance-to-ground of the wire attached to it is increased by several tenths of a micromicrofarad. While attempts have been made to overcome this drawback, they have left much to be desired. For example, some improvement was attained by balancing one wire, or combination of wires, against another wire or combination of wires. Such arrangements were satisfactory when the system was dry or wet but proved to be unsatisfactory during the period the antenna was in the process of being wetted by rain.

I have found that such outdoor intrusion detection systems may be rendered independent of wetting by rain so as to avoid false alarms or undesired desensitization of the system.

It is, therefore, an object of this invention to provide an improved outdoor intrusion detection system wherein an antenna wire is utilized to sense the presence of an intruder and which, for all practical purposes, is insensitive to the effect of rain.

A more specific object is to provide such a system wherein the antenna wire though exposed to being wetted by rain is mounted from its supports in such manner that the change in coupling between the antenna and ground or between the antenna wires when the antenna is wetted by rain is kept so small as to be negligible.

Other objects as well as advantages of this invention will be apparent from the following description and the accompanying drawing in which:

Figure l is a diagrammatic view of an intrusion detection system including an outdoor antenna;

Figure 2 is a perspective view of a portion of an antenna installation;

Figure 3 is an end view, on an enlarged scale, of an antenna wire support assembly; and

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Figure 4 is a cross-sectional view through the line 4-4 of Figure 3.

Referring now to the drawings in detail, an intrusion detection system indicated diagrammatically at 10 may include an outdoor antenna installation 11 in the form of a perimeter wire included in the tank circuit of an oscillator 12 so that the capacitance-to-ground of the antenna affects the frequency of the oscillator. As shown in U. S. Patent No. 2,343,987 or Patent No. 2,455,376, the oscillator may feed a tuned circuit 13, the output from which varies with the oscillator frequency and serves, upon the occurrence of a predetermined change in antenna circuit current, to energize a relay 14 which in turn serves to control well known alarm devices. if desired the antenna may also include connections to tuned circuit 13, as for example in accordance with said Patent No. 2,455,376.

At each point where the antenna wire 15 is supported from a post 16, an inverted, cup-shaped conductive shield 17 is provided and connected to its support post 16 by means of a stand-0E support bracket 18 or the like. An insulator 19 which may be porcelain or of other suitable material is connected to the upwardly presented closed end of shield 17 and carries a downwardly extending post 20 centrally located within the shield.

Slots are provided in each of the shields 17 to provide a through passage for the antenna wire 15. In the present instance a narrow slot 21 is formed in one side of shield 17 and is sulficiently wide to permit antenna wire 15 to pass freely therethrough. Opposite slot 21 a relatively large portion of the shield 17 is cut away to provide a side opening which is partially closed by an arcuate member 23 having a slot 24 formed therethrough. Members 23 are each connected to their respective shields 17 as indicated at 25 and are provided with horizontal slots 26 to permit angular adjustment of the relationship between slot 24 and slot 21 in each of the shields 17 whereby the shields may be readily adjusted to accommodate relatively wide departures from a relationship between the entering and exiting wires 15. The antenna wire 15 is connected to post 20 in any suitable Way but completely within each of the shields 17.

In order to prevent water which collects on the exposed wire from running and reaching the insulators, a pair of disc-like elements such as apertured grommets 27 which may be of rubber or other suitable insulative material are mounted on wire 15, one just inside each of the slots 21, 24.

The shields 17, which may readily be fabricated from metal having good weathering properties, are conveniently grounded electrically by being conductively connected through metal brackets 18 to metal support posts 16 which in turn are conductive and grounded in the earth.

It is important that such installations combine with required sensitivity a high degree of stability to insure that alarms are properly initiated with a minimum of false alarms. Obviously, if a change in the wire to ground capacitance of several micromicrofarads were to occur as hitherto at the antenna wire support points whenever it rains, the system must either be desensitized to that extent or rebalanced as the antenna is wetted.

However, in the present arrangement, insulators 19 are effectively maintained dry even though the antenna system is exposed to rain and, as a consequence, there is no change in capacitance across the insulators. Since the metal shields 17 are grounded they present the same capacitance to the antenna wire whether Wet or dry. The only change in capacitance due to wetting of the antenna appurtenances which occurs during rain is that between the small segmen o ant nna wire intermediate each of the grommets 27 and the adjacent portions of the metal shield 17. This amounts to a change in capacitance of about 0.005 micromicrofarads at each shield or an improvement by a factor of the order of 100 over arrangements used hitherto.

It should also be noted that such changes in coupling between antenna wires or between the antenna wires and ground as may occur are retarded and may occur only over a substantially longer time than hitherto. As a consequence, there is additional assurance against the transmission of a false alarm.

The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention, in the use of such terms and expressions, of excluding any equivalents of the features shown and described or portions thereof, but it is recognized that various modifications are possible within the scope of the invention claimed.

I claim:

1. In an intrusion detection system in which a change in the electrical coupling of an outdoor antenna is utilized to sense the presence of an intruder, an antenna wire, a plurality of supports for said antenna wire each including a conductive, cup-shaped shield, means supporting and electrically grounding said shield with its open end presented downwardly, said shield having a slot formed through one side thereof and an opening formed through the opposite side thereof substantially wider than said slot, an arcuate member adjustably connected to said shield for circumferential movement relative thereto and having a slot formed therethrough in registration with said opening whereby said last mentioned slot may be shifted circumferentially about said shield and relative to said opening, an insulator connected to and within the enclosure formed by said shield, a post connected to said insulator and supported thereby in insulated relation with said shield, means connecting said antenna Wire to said post, and a pair of disc-like elements mounted on said antenna wire Within said shield with one adjacent to each of said slots and spaced from said post.

2. In an intrusion detection system in which a change in the electrical coupling of an outdoor antenna is utilized to sense the presence of an intruder, an antenna wire, a plurality of supports for said antenna wire each including a conductive, cup-shaped shield, means supporting and electrically grounding said shield with its open end presented downwardly, said shield having a slot formed through one side thereof and an opening formed through the opposite side thereof substantially wider than said slot, an arcuate member adjustably connected to said shield for circumferential movement relative thereto and having a slot formed therethrough in registration with said opening whereby said last mentioned slot may be shifted circumferentially about said shield and relative to said opening, an insulator connected to and within the enclosure formed by said shield, a post connected to said insulator and supported thereby in insulated relation with said shield, and means connecting said antenna wire to said post.

References Cited in the file of this patent UNITED STATES PATENTS 

